The present invention relates to the generation of charged particles, and more particularly to the high-speed generation of charged particle images for electrographic imaging.
Charged particles for use in electrographic imaging can be generated in a wide variety of ways. Common techniques include the use of air gap breakdown, corona discharges and spark discharges. Other techniques employ triboelectricity, radiation, and microwave breakdown. When utilized for the formation of latent electrostatic images, all of the above techniques suffer certain limitations in charged particle output currents and charge image integrity.
A further approach, which offers significant advantages in this regard, is described in Fotland, U.S. Pat. No. 4,155,093 (May 19, 1979) and the improvement disclosed in Carrish, U.S. Pat. No. 4,160,257 (Jul. 3, 1979). These patents disclose a method and apparatus for generating charged particles in air involving what the inventors' term "silent electric discharge". The prior art general view of FIG. 1 shows an image generating printhead, 8 capable of forming an electrostatic latent image on dielectric receptor 25. The printhead 8 is supplied with a high voltage alternating potential 10 applied between two electrodes; i.e., generator electrode 12 and control electrode 14. Electrode 14 contains a plurality of circular or slotted apertures opposing generator electrode 12. A solid dielectric member 16 separates these electrodes. Driver electrode 12 is shown encapsulated by dielectric 18. As disclosed in U.S. Pat. No. 4,155,093, the alternating potential causes the formation of a pool or plasma of positive and negative charged particles in an air region adjacent the dielectric, which charged particles may be extracted to form a latent electrostatic image. The alternating potential 10 creates a fringing field between the two electrodes and, when the electrical stress exceeds the dielectric strength of air, a discharge occurs quenching the field. Such silent electric discharges cause a faint blue glow.
U.S. Pat. No. 4,160,257 teaches the use of an isolation electrode, 20, separated from the control electrode 14 by a dielectric spacer layer 22. Electrode 20 serves to screen the extraction electric fields in the region bounded by electrodes 14 and 20 from the external fields of the latent image. In addition, the aperture 24 in electrode 20 provides an electrostatic lensing action. Charged particles are permitted to pass through the isolation aperture 24 to the surface of the image receptor 25. The image receptor dielectric layer 26 is contiguous with conducting substrate 28.
The use of negative charges (electrons and negative ions) is preferred since higher negative output currents are obtained than when employing positive charges. Biasing power supply 34 is used to provide a high-voltage accelerating field between dielectric receptor substrate 28 and isolation electrode 20. Negative charges are extracted from the discharge when the print selector switch 36 is in position Y. In this case, a charge extraction field, provided by power supply 30, is present between electrodes 14 and 20. With the switch in position X, a retarding field is applied by supply 32 and no charge may escape aperture 24.
The requirement that both a high frequency voltage and an extraction voltage be simultaneously present to generate a charge output provides the means for coincident selection so that the charge latent image generator may be multiplexed. As seen in the prior art view of FIG. 2, the charged particle generators of the above-discussed patents may be embodied in a multiplexed print head 40, wherein an array of control electrodes 50-1 through 50-6 contain holes or slots 45 at crossover regions opposite generator electrodes 60-1 through 60-4. Dielectric layer 41 isolates generator and driver electrodes. Driver electrodes are sequentially excited by a high frequency high voltage burst of cycles, and any location in the matrix may be printed by applying a data, or control, pulse to the appropriate control electrode at the time that the corresponding generator line is excited.
This basic scheme of multiplexing has been extensively employed in print heads having resolutions ranging from about 7.8 dots per millimeter to 23.6 dots per millimeter with the number of generator lines ranging from 12 to 21. Since the human eye is quite sensitive to periodic optical density variations in this spatial frequency region near one cycle per millimeter, the periodic control line repeat of about one-millimeter can lead to an observable fixed streak pattern in the direction of paper travel below printhead 8. There are three potential sources for this problem. An error in the print head mounting will cause a periodic variation in pixel spacing since the print head array is spread over a two dimensional pattern. Secondly, a variation in print head to dielectric receptor spacing naturally occurs when printing from a planar print head onto a cylindrical dielectric receptor surface. The reduced field at the extreme ends of the print head then results in lower output current in these regions. Finally, as the image is laid down there has to be a last down pixel whose size is affected by the fringing field of neighboring pixels.
These defects are minimized by using care in the design of the print head mounting mechanism, printing onto a large radius dielectric receptor or designing overhang compensation into the print head, and by arranging the print head geometry such that the pixels are interleaved. This last improvement decreases the pitch of the fixed pattern noise from about one-millimeter to one-half of the print head aperture spacing.
A number of U.S. patents have issued which disclose improved methods of manufacture or geometry and two of these disclose variations of multiplexing geometry which provide interleaving in the manner in which the dots are placed on the surface of the dielectric receptor. McCallum, U.S. Pat. No. 4,999,653 (Mar. 12, 1991) discloses a control line geometry that is staggered to provide for interleaving of the printed dots. Buchan, U.S. Pat. No. 5,006,869 (Apr. 9, 1991) solves this problem by etching the generator lines in the form of a chevron or by having each control line contain staggered apertures.
Charge latent image formation operating speeds are limited both by the number of available charging cycles each pixel requires and by the current output of the print head. The speed limitation imposed may be calculated from the following formula: EQU S=f/(cnr) meters/second
Wherein
S=maximum printing speed PA1 f=operating frequency of generator oscillator PA1 c=number of cycles required to print a pixel PA1 n=number of generator electrodes in the print head PA1 r=resolution of print head
Taking, for example, an oscillator frequency of 5 megahertz, a cycle requirement of 6 cycles to obtain good image density, 22 generator electrodes, and a resolution of 23,600 apertures per meter, the maximum print speed is calculated to be 1.6 meters per second. The maximum spacing between adjacent control electrodes is approximately equal to the number of generator lines divided by the distance between adjacent resolution elements. For the above example, this spacing is 0.93 millimeters. The true distance between adjacent control electrodes is actually somewhat smaller since the control electrodes are slightly angled with respect to the direction of printhead motion. With sufficient spacing between generator lines, the actual spacing is relatively close to that given above.
The high frequency discharge in air results in the formation of minute quantities of nitric acid, active oxygen, and other highly corrosive compounds. In addition, the high frequency operation results in the generation of appreciable quantities of heat in the very small regions at the discharge site in and around the generator electrode apertures. For these reasons, it is desirable to form the generator electrode from corrosion resistant metal foils having a thickness of at least about 12 microns. Stainless steel foil is widely employed in this application. Reduced corrosion rates are realized with the use of very corrosion resistant materials such as molybdenum, tungsten, or tantalum. These materials are photo-etched by coating or laminating a photosensitive resist to the foil. The resist is exposed, developed, and the foil then chemically etched. It is difficult to etch complex structures in these corrosion resistant films with precision at very close spacing, particularly since each control electrode must contain a plurality of etched holes. Furthermore, the close spacing places a very high thermal load in a very small area. A conflict exits, therefore, between operation at high speeds and available resolution. For these reasons, present commercially available print heads are limited to a maximum resolution of 12 dots per millimeter using 12 generator electrodes and 24 dots per millimeter at 19 generator electrodes.
In order to generate the high output currents required of high-speed operation, it is desirable to etch the apertures to diameters of about 0.15 mm. The edges of the control electrodes must be sealed with a dielectric in order to eliminate air breakdown at these edges. This constraint also leads to a requirement for control line spacing of about 0.9 mm or greater.
Operation at frequencies over five megahertz results in high power density loading in the printhead as well as difficulties involving excessive RF emission. Although frequencies as high as ten megahertz have been employed in the laboratory, five-megahertz is the highest frequency that has been employed in commercial printers.
Accordingly, it is a principal object of the invention to provide an improved easily manufactured charge image generator capable of operating at high speeds. A further objective of the invention is to simplify fabrication of the charge image generator apparatus. A still further objective of the invention is to minimize artifacts in the charge image caused by periodic variations in the matrix array. Related objectives are to reduce the power density loading of the print head and also to provide for interleaved dot operation in order to avoid fixed pattern noise.